From ae4eabc96078d0d627418d99ca146de3ae295c86 Mon Sep 17 00:00:00 2001 From: Jay Bryant Date: Fri, 6 Oct 2017 14:19:23 -0500 Subject: [PATCH] Editing pass for retry.adoc I made it more readable and consistent and fixed up sentence errors. I added a TODO for a spot that could benefit from an example or two. polishing on merge --- spring-batch-docs/asciidoc/retry.adoc | 181 +++++++++++++------------- 1 file changed, 90 insertions(+), 91 deletions(-) diff --git a/spring-batch-docs/asciidoc/retry.adoc b/spring-batch-docs/asciidoc/retry.adoc index 57560994e..4c7e6a2fa 100644 --- a/spring-batch-docs/asciidoc/retry.adoc +++ b/spring-batch-docs/asciidoc/retry.adoc @@ -6,10 +6,17 @@ == Retry +To make processing more robust and less prone to failure, it sometimes + helps to automatically retry a failed operation in case it might + succeed on a subsequent attempt. Errors that are susceptible to intermittent failure + are often transient in nature. Examples include remote calls to a web + service that fails because of a network glitch or a + `DeadlockLoserDataAccessException` in a database update. + [[retryTemplate]] -=== RetryTemplate +=== `RetryTemplate` [NOTE] @@ -19,16 +26,9 @@ The retry functionality was pulled out of Spring Batch as of 2.2.0. ==== -To make processing more robust and less prone to failure, sometimes - it helps to automatically retry a failed operation in case it might - succeed on a subsequent attempt. Errors that are susceptible to this kind - of treatment are transient in nature. For example a remote call to a web - service or RMI service that fails because of a network glitch or a - `DeadlockLoserDataAccessException` in a database update may - resolve themselves after a short wait. To automate the retry of such +To automate retry operations Spring Batch has the `RetryOperations` - strategy. The `RetryOperations` interface looks like - this: + strategy. The following interface definition for `RetryOperations`: [source, java] @@ -49,8 +49,8 @@ public interface RetryOperations { } ---- -The basic callback is a simple interface that allows you to - insert some business logic to be retried: +The basic callback is a simple interface that lets you + insert some business logic to be retried, as shown in the following interface definition: [source, java] @@ -62,18 +62,18 @@ public interface RetryCallback { } ---- -The callback is executed and if it fails (by throwing an - `Exception`), it will be retried until either it is - successful, or the implementation decides to abort. There are a number of +The callback runs and, if it fails (by throwing an + `Exception`), it is retried until either it is + successful or the implementation aborts. There are a number of overloaded `execute` methods in the - `RetryOperations` interface dealing with various use - cases for recovery when all retry attempts are exhausted, and also with + `RetryOperations` interface. Those methods deal with various use + cases for recovery when all retry attempts are exhausted and deal with retry state, which allows clients and implementations to store information - between calls (more on this later). + between calls (we cover this in more detail later in the chapter). The simplest general purpose implementation of `RetryOperations` is - `RetryTemplate`. It could be used like this + `RetryTemplate`. It can be used as follows: [source, java] @@ -95,21 +95,21 @@ Foo result = template.execute(new RetryCallback() { }); ---- -In the example we execute a web service call and return the result - to the user. If that call fails then it is retried until a timeout is +In the preceding example, we make a web service call and return the result + to the user. If that call fails, then it is retried until a timeout is reached. [[retryContext]] -==== RetryContext +==== `RetryContext` The method parameter for the `RetryCallback` - is a `RetryContext`. Many callbacks will simply - ignore the context, but if necessary it can be used as an attribute bag + is a `RetryContext`. Many callbacks + ignore the context, but, if necessary, it can be used as an attribute bag to store data for the duration of the iteration. -A `RetryContext` will have a parent context +A `RetryContext` has a parent context if there is a nested retry in progress in the same thread. The parent context is occasionally useful for storing data that need to be shared between calls to `execute`. @@ -117,13 +117,13 @@ A `RetryContext` will have a parent context [[recoveryCallback]] -==== RecoveryCallback +==== `RecoveryCallback` -When a retry is exhausted the +When a retry is exhausted, the `RetryOperations` can pass control to a different - callback, the `RecoveryCallback`. To use this - feature clients just pass in the callbacks together to the same method, - for example: + callback, called the `RecoveryCallback`. To use this + feature, clients pass in the callbacks together to the same method, + as shown in the following example: [source, java] @@ -148,7 +148,7 @@ If the business logic does not succeed before the template ==== Stateless Retry -In the simplest case, a retry is just a while loop: the +In the simplest case, a retry is just a while loop. The `RetryTemplate` can just keep trying until it either succeeds or fails. The `RetryContext` contains some state to determine whether to retry or abort, but this @@ -157,7 +157,7 @@ In the simplest case, a retry is just a while loop: the stateless and stateful retry is contained in the implementation of the `RetryPolicy` (the `RetryTemplate` can handle both). In a stateless - retry, the callback is always executed in the same thread on retry as + retry, the retry callback is always executed in the same thread it was on when it failed. [[statefulRetry]] @@ -169,23 +169,23 @@ Where the failure has caused a transactional resource to become invalid, there are some special considerations. This does not apply to a simple remote call because there is no transactional resource (usually), but it does sometimes apply to a database update, especially when using - Hibernate. In this case it only makes sense to rethrow the exception - that called the failure immediately so that the transaction can roll - back and we can start a new valid one. + Hibernate. In this case it only makes sense to re-throw the exception + that called the failure immediately, so that the transaction can roll + back and we can start a new, valid transaction. -In these cases a stateless retry is not good enough because the +In cases involving transactions, a stateless retry is not good enough, because the re-throw and roll back necessarily involve leaving the `RetryOperations.execute()` method and potentially losing the context that was on the stack. To avoid losing it we have to introduce a storage strategy to lift it off the stack and put it (at a minimum) in - heap storage. For this purpose Spring Batch provides a storage strategy - `RetryContextCache` which can be injected into the + heap storage. For this purpose, Spring Batch provides a storage strategy called + `RetryContextCache`, which can be injected into the `RetryTemplate`. The default implementation of the `RetryContextCache` is in memory, using a simple `Map`. Advanced usage with multiple processes in a clustered environment might also consider implementing the `RetryContextCache` with a cluster cache of some - sort (though, even in a clustered environment this might be + sort (however, even in a clustered environment, this might be overkill). Part of the responsibility of the @@ -194,14 +194,14 @@ Part of the responsibility of the in a new transaction). To facilitate this, Spring Batch provides the `RetryState` abstraction. This works in conjunction with a special `execute` methods in the - `RetryOperations`. + `RetryOperations` interface. The way the failed operations are recognized is by identifying the state across multiple invocations of the retry. To identify the state, - the user can provide an `RetryState` object that is + the user can provide a `RetryState` object that is responsible for returning a unique key identifying the item. The identifier is used as a key in the - `RetryContextCache`. + `RetryContextCache` interface. [WARNING] @@ -210,13 +210,13 @@ Be very careful with the implementation of `Object.equals()` and `Object.hashCode()` in the key returned by `RetryState`. The best advice is to use a business key to identify the items. In the case of a JMS - message the message ID can be used. + message, the message ID can be used. ==== -When the retry is exhausted there is also the option to handle the +When the retry is exhausted, there is also the option to handle the failed item in a different way, instead of calling the - `RetryCallback` (which is presumed now to be likely + `RetryCallback` (which is now presumed to be likely to fail). Just like in the stateless case, this option is provided by the `RecoveryCallback`, which can be provided by passing it in to the `execute` method of @@ -224,28 +224,28 @@ When the retry is exhausted there is also the option to handle the The decision to retry or not is actually delegated to a regular `RetryPolicy`, so the usual concerns about limits - and timeouts can be injected there (see below). + and timeouts can be injected there (described later in this chapter). [[retryPolicies]] === Retry Policies -Inside a `RetryTemplate` the decision to retry +Inside a `RetryTemplate`, the decision to retry or fail in the `execute` method is determined by a - `RetryPolicy` which is also a factory for the + `RetryPolicy`, which is also a factory for the `RetryContext`. The `RetryTemplate` has the responsibility to use the current policy to create a `RetryContext` and pass that in to the `RetryCallback` at every attempt. - After a callback fails the `RetryTemplate` has to + After a callback fails, the `RetryTemplate` has to make a call to the `RetryPolicy` to ask it to update - its state (which will be stored in the - `RetryContext`), and then it asks the policy if - another attempt can be made. If another attempt cannot be made (e.g. a + its state (which is stored in the + `RetryContext`) and then asks the policy if + another attempt can be made. If another attempt cannot be made (such as when a limit is reached or a timeout is detected) then the policy is also - responsible for handling the exhausted state. Simple implementations will - just throw `RetryExhaustedException` which will cause + responsible for handling the exhausted state. Simple implementations + throw `RetryExhaustedException`, which causes any enclosing transaction to be rolled back. More sophisticated implementations might attempt to take some recovery action, in which case the transaction can remain intact. @@ -253,27 +253,26 @@ Inside a `RetryTemplate` the decision to retry [TIP] ==== -Failures are inherently either retryable or not - if the same +Failures are inherently either retryable or not. If the same exception is always going to be thrown from the business logic, it - doesn't help to retry it. So don't retry on all exception types - try to - focus on only those exceptions that you expect to be retryable. It's not + does no good to retry it. So do not retry on all exception types. Rather, try to + focus on only those exceptions that you expect to be retryable. It is not usually harmful to the business logic to retry more aggressively, but - it's wasteful because if a failure is deterministic there will be time - spent retrying something that you know in advance is fatal. + it is wasteful, because, if a failure is deterministic, you spend time + retrying something that you know in advance is fatal. ==== Spring Batch provides some simple general purpose implementations of - stateless `RetryPolicy`, for example a - `SimpleRetryPolicy`, and the - `TimeoutRetryPolicy` used in the example - above. + stateless `RetryPolicy`, such as + `SimpleRetryPolicy` and + `TimeoutRetryPolicy` (used in the preceding example). -The `SimpleRetryPolicy` just allows a retry on +The `SimpleRetryPolicy` allows a retry on any of a named list of exception types, up to a fixed number of times. It also has a list of "fatal" exceptions that should never be retried, and this list overrides the retryable list so that it can be used to give - finer control over the retry behavior: + finer control over the retry behavior, as shown in the following example: [source, java] @@ -301,13 +300,13 @@ There is also a more flexible implementation called user to configure different retry behavior for an arbitrary set of exception types though the `ExceptionClassifier` abstraction. The policy works by calling on the classifier to convert an - exception into a delegate `RetryPolicy`, so for + exception into a delegate `RetryPolicy`. For example, one exception type can be retried more times before failure than another by mapping it to a different policy. Users might need to implement their own retry policies for more - customized decisions. For instance, if there is a well-known, - solution-specific, classification of exceptions into retryable and not + customized decisions. For instance, a custom retry policy makes sense when there is a well-known, + solution-specific classification of exceptions into retryable and not retryable. [[backoffPolicies]] @@ -315,13 +314,14 @@ Users might need to implement their own retry policies for more === Backoff Policies -When retrying after a transient failure it often helps to wait a bit +When retrying after a transient failure, it often helps to wait a bit before trying again, because usually the failure is caused by some problem - that will only be resolved by waiting. If a + that can only be resolved by waiting. If a `RetryCallback` fails, the `RetryTemplate` can pause execution according to the - `BackoffPolicy` in place. + `BackoffPolicy`. +The following code shows the interface definition for the `BackOffPolicy` interface: [source, java] ---- @@ -339,8 +339,8 @@ A `BackoffPolicy` is free to implement the backOff in any way it chooses. The policies provided by Spring Batch out of the box all use `Object.wait()`. A common use case is to backoff with an exponentially increasing wait period, to avoid two retries - getting into lock step and both failing - this is a lesson learned from - the ethernet. For this purpose Spring Batch provides the + getting into lock step and both failing (this is a lesson learned from + ethernet). For this purpose, Spring Batch provides the `ExponentialBackoffPolicy`. [[retryListeners]] @@ -348,16 +348,16 @@ A `BackoffPolicy` is free to implement === Listeners -Often it is useful to be able to receive additional callbacks for +Often, it is useful to be able to receive additional callbacks for cross cutting concerns across a number of different retries. For this - purpose Spring Batch provides the `RetryListener` - interface. The `RetryTemplate` allows users to - register `RetryListeners`, and they will be given - callbacks with the `RetryContext` and + purpose, Spring Batch provides the `RetryListener` + interface. The `RetryTemplate` lets users + register `RetryListeners`, and they are given + callbacks with `RetryContext` and `Throwable` where available during the iteration. -The interface looks like this: +The following code shows the interface definition for `RetryListener`: [source, java] @@ -374,35 +374,35 @@ public interface RetryListener { The `open` and `close` callbacks come before and after the entire - retry in the simplest case and `onError` applies to + retry in the simplest case, and `onError` applies to the individual `RetryCallback` calls. The `close` method might also receive a - `Throwable`; if there has been an error it is the + `Throwable`. If there has been an error, it is the last one thrown by the `RetryCallback`. -Note that when there is more than one listener, they are in a list, - so there is an order. In this case `open` will be +Note that, when there is more than one listener, they are in a list, + so there is an order. In this case, `open` is called in the same order while `onError` and - `close` will be called in reverse order. + `close` are called in reverse order. [[declarativeRetry]] === Declarative Retry -Sometimes there is some business processing that you know you want +Sometimes, there is some business processing that you know you want to retry every time it happens. The classic example of this is the remote service call. Spring Batch provides an AOP interceptor that wraps a method - call in a `RetryOperations` for just this purpose. + call in a `RetryOperations` implementation for just this purpose. The `RetryOperationsInterceptor` executes the intercepted method and retries on failure according to the `RetryPolicy` in the provided `RepeatTemplate`. -Here is an example of declarative iteration using the Spring AOP +The following example shows a declarative iteration that uses the Spring AOP namespace to repeat a service call to a method called `remoteCall` (for more detail on how to configure - AOP interceptors see the Spring User Guide): + AOP interceptors, see the Spring User Guide): [source, xml] @@ -418,8 +418,7 @@ Here is an example of declarative iteration using the Spring AOP class="org.springframework.batch.retry.interceptor.RetryOperationsInterceptor"/> ---- -The example above uses a default +The preceding example uses a default `RetryTemplate` inside the interceptor. To change the - policies or listeners, you only need to inject an instance of + policies or listeners, you can inject an instance of `RetryTemplate` into the interceptor. -